The Cape Plumbago (Plumbago auriculata) is a popular, sprawling shrub native to South Africa, prized for its nearly continuous display of sky-blue flowers from spring through fall. This vigorous growth requires a consistent supply of nutrients to maintain prolific blooming. While the plant is relatively low-maintenance, providing the right fertilizer ensures healthy foliage and maximum bloom production.
Selecting the Optimal Fertilizer Formula
The choice of fertilizer depends on whether the goal is balanced growth or maximum floral output. General-purpose formulas, such as a balanced 10-10-10 or 8-8-8 NPK ratio, are suitable for maintaining overall plant health during the growing season. This equal distribution of macronutrients supports strong root development and lush, green foliage.
To maximize the production of blue flowers, a formula with a higher proportion of phosphorus is recommended. Phosphorus, the middle number in the NPK ratio, directly supports flower bud formation and bloom quality. A “bloom-boosting” fertilizer, such as one with a 15-30-15 ratio, provides the necessary energy to fuel continuous flowering throughout the warm months.
Both granular and liquid fertilizers offer distinct advantages. Slow-release granular fertilizer provides a steady food supply over several months, minimizing the need for frequent application. Liquid, water-soluble fertilizers offer a quick nutrient boost, making them ideal for supplementing granular applications or for plants showing immediate signs of nutrient stress.
Plumbago prefers slightly acidic to neutral soil, typically a pH range between 5.0 and 7.0. If the soil is alkaline (high pH), which inhibits the plant’s ability to absorb certain micronutrients, an acid-forming fertilizer may be beneficial. Look for products that include secondary nutrients and micronutrients, particularly iron and manganese, as these prevent common deficiency symptoms.
Scheduling and Application Techniques
Primary feeding should occur in early spring, just as the Plumbago shows signs of new growth after winter dormancy. Applying a slow-release granular fertilizer at this time provides a consistent nutrient supply for the initial months of the active growing period. Granular products should be spread evenly around the root zone, avoiding the main stem, and lightly worked into the top inch of soil before being thoroughly watered in.
Following the initial spring application, supplemental feeding should be maintained throughout the blooming period. For plants receiving slow-release granular feed, this may mean a second application in late summer or early fall, or a transition to liquid fertilizer. If using a balanced liquid fertilizer, apply it every four to six weeks from late spring through early fall.
When utilizing a concentrated liquid formula, it is important to follow the product’s dilution instructions precisely to prevent root burn. A common dilution rate is approximately one-quarter teaspoon per gallon of water. Always ensure the plant is well-hydrated before applying any fertilizer, especially liquid concentrates or fast-acting granular types, as dry roots are highly susceptible to chemical burn. All feeding should cease by late fall to allow the plant to harden off and prepare for its rest period.
Recognizing and Addressing Nutrient Deficiencies
Plumbago can exhibit signs of specific nutritional deficiencies, often related to soil alkalinity. A lack of nitrogen, the nutrient responsible for green, leafy growth, typically presents as a uniform, pale yellowing across the older, lower leaves. This indicates the plant is moving nitrogen from old foliage to support new development, requiring a quick boost from a high-nitrogen liquid feed.
A more frequent issue for Plumbago in high-pH soil is iron deficiency, known as iron chlorosis. This is visually distinct because the newest leaves at the tips of the branches turn yellow while the veins remain green. Although iron is present in the soil, alkaline conditions prevent the plant from absorbing it efficiently.
To correct iron chlorosis, applying a soil acidifier or a chelated iron supplement is the most direct approach. Chelated iron is formulated to remain available for root uptake even in alkaline soil conditions, providing a rapid solution to restore green coloration. If the issue is manganese deficiency, the symptoms are similar, but a specific application of manganese sulfate is necessary to correct the imbalance.